1. Field of the Invention
The present invention relates in general to a shift control apparatus for a vehicle step-variable automatic transmission having a plurality of selectable groups of forward-drive positions having respective different sets of speed ratios, and more particularly to techniques associated with the shift control apparatus, for improving the drivability of a vehicle by suitably selecting one of the groups of forward-drive positions of the automatic transmission, which enables the vehicle to be driven with an adequate drive force with the automatic transmission placed in a relatively high-speed forward-drive position.
2. Discussion of Related Art
A known shift control apparatus for a vehicle step-variable automatic transmission tends to suffer from frequent alternate shift-up and shift-down actions of the automatic transmission for repeated alternate acceleration and deceleration of the vehicle so as to maintain the vehicle speed at a constant value while a relatively large vehicle drive force is required with the automatic transmission placed in a relatively high-speed forward-drive position, for example, while the vehicle is running on an uphill roadway. The frequent shift-up and shift-down actions of the automatic transmission are felt uncomfortable by the vehicle operator. A vehicle transmission shift control apparatus to overcome this drawback is proposed on pages 2–18 and 2–19 of Toyota “CELSIOR” New Model Instruction Manual, August 2000, published by the Service Department of Toyota Jidosha Kabushiki Kaisha. This shift control apparatus is arranged to detect an uphill running of the vehicle on the basis of the running speed and an opening angle of a throttle valve, and restrict a shift-up action of the automatic transmission to a relatively high-speed forward-drive position during the detected uphill running of the vehicle, so as to enable the vehicle to be driven with a sufficient drive force while restricting frequent shifting actions of the automatic transmission.
The step-variable automatic transmission has only one group of forward-drive positions which have respective different speed ratios, and the shift control apparatus disclosed in the above-identified Instruction Manual is arranged to restrict or inhibit a shift-up action of the automatic transmission to the highest-speed forward-drive position, for example, to the fifth-speed position having a speed ratio of 0.753, during the uphill running of the vehicle. In this case, the fourth-speed position having a speed ratio of 1.000, for example, is used in place of the fifth-speed position. Since the fifth-speed and fourth-speed positions have a considerably large difference in the speed ratio, the use of the fourth-speed position in place of the fifth-speed position during the uphill running of the vehicle is also felt uncomfortable by the vehicle operator in respect of the vehicle acceleration and deceleration characteristics. Further, the restriction of the shift-up action of the automatic transmission to the highest-speed position has a high risk of deterioration of the fuel economy of the vehicle.